The present invention relates to a process for specifically cleaving proteins such as fusion proteins derived from genetically engineered microorganisms.
In the synthesis of foreign proteins (e.g., mammal proteohormones) by genetically modified microorganisms, the foreign gene which codes for the desired protein sequence is incorporated into a structural gene of the microorganism. Concomitantly, the regulation sites on the microbial structural gene remain functional; thus, protein biosynthesis can occur in the usual way from the microbial starting codon for methionine to the stop codon on the foreign gene.
As a result of this protein manufacture, a fusion protein is obtained as the primary product. It contains at the amino terminal end, a more or less long sequence of the microbial indigenous protein, in the special case of the so-called "direct synthesis" only the starting amino acid methionine, and the carboxy terminal end, the desired foreign protein. For its subsequent use, the foreign protein must first be processed by a specific cleavage from this fusion protein. The only method known for this cleavage at the present time is a reaction with cyanogen bromide which leads to a cleavage of the peptide sequence at the carboxy end of methionine residues (S. B. Needleman, "Protein Sequence Determination", Springer Publishers, 1970, N.Y.). Accordingly, it is necessary for this purpose that the foreign gene, at the 5'-end of the codegenic strand, contain an additional codon for methionine, whereby a methionine residue is disposed between the N-terminal native protein sequence and the foreign protein of the fusion protein. This method, however, fails if other methionine residues are present in the desired foreign protein per se. Additionally, the cleavage with cyanogen bromide has the disadvantage of evoking secondary reactions at various other amino acids.